In order to improve device performance, semiconductor device circuit densities continue to increase. This increase in circuit density is realized by decreasing feature sizes. Current technologies target feature sizes of 0.15 μm and 0.13 μm with further decreases expected in the near future.
The exact dimensions of features within the devices are controlled by all steps in the fabrication process. Vertical dimensions are controlled by doping and layering processes where as horizontal dimensions are determined primarily by photolithographic processes. The horizontal widths of the lines and spaces that make up the circuit patterns are often referred to as critical dimensions (CD).
Photolithography is the technique used to form the precise circuit patterns on the substrate surface. These patterns are transferred into the wafer structure by a subsequent etch or deposition process. Ideally, the photolithography step creates a pattern that exactly matches the design dimensions (correct CD) at the designed locations (known as alignment or registration).
Photolithography is a multi-step process where the desired pattern is first formed on a photomask (or reticle). The pattern is transferred to the substrate through a photomasking operation where radiation (e.g., UV light) is transmitted through the patterned photomask exposing a radiation sensitive coating on the substrate. This coating (photoresist) undergoes a chemical change upon exposure to the radiation which renders the exposed areas either more or less soluble to a subsequent develop chemistry. Photolithography techniques are well known in the art. An overview of these techniques can be found in the text of Introduction to Microlithography edited by Thompson et al.
Since the photomask acts as the master for generating the circuit patterns on a large number of substrates, any imperfections introduced during the manufacturing of the photomask will be replicated on all wafers imaged with that photomask. Consequently, fabricating a high quality photomask that faithfully represents the designed patterns and dimensions is critical to creating high yield device manufacturing processes.
There are two major types of photomask reticles that are well known in the art: absorber and phase shifting. An absorber photomask typically consists of an optically transparent substrate (e.g., fused-quartz, CaF2, etc.) that is coated with an opaque film (e.g., Cr). The opaque film may consist a single layer or multiple materials (e.g., an anti-reflective layer (AR chromium) on top of an underlying chromium layer). In the case of binary chromium photomasks, examples of commonly used opaque films (listed by trade name) include, but are not limited to, AR8, NTAR7, NTAR5, TF11, TF21. During fabrication of the photomask, the opaque film is deposited on the transparent substrate. A resist layer is then deposited on top of the opaque layer and patterned (e.g., exposure to a laser or electron beam). Once exposed, the resist layer is then developed exposing areas of the underlying opaque film to be removed. A subsequent etch operation removes the exposed film forming the absorber photomask.
There are two subcategories of phase shifting masks that are well known in the art: alternating and embedded attenuating masks. Alternating phase shift masks typically consist of an optically transparent substrate (e.g., fused-quartz, CaF2, etc.) that is coated with an opaque film (e.g., Cr and antireflective Cr). During fabrication of the photomask, the opaque film is deposited on the transparent substrate. A resist layer is then deposited on top of the opaque layer and patterned using a laser or electron beam. Once exposed, the resist layer is then developed exposing areas of the underlying opaque film to be removed. An etch process removes the exposed opaque film exposing the underlying substrate. A second process is used to etch a precise depth into the underlying substrate. Optionally, the substrate may be subjected to a second resist coat and develop process prior to the second etch process as is known in the art.
Embedded attenuating phase shift masks (EAPSM) typically consist of an optically transparent substrate (e.g., fused-quartz, CaF2, etc.) that is coated with a film or stack of films that attenuate the transmitted light while shifting the phase 180 degrees at a desired wavelength. An opaque film or film stack (e.g., Cr and antireflective Cr) is then deposited on the phase shift material. A resist layer is then deposited on top of the opaque layer and patterned (e.g., using a laser or electron beam). Once exposed, the resist layer is then developed exposing areas of the underlying opaque film to be removed. An etch process is then used to remove the exposed opaque film exposing the underlying phase shifting/attenuating film or film stack. Following etching the opaque film, a second etch process is used to etch the phase shift layer stopping on the underlying substrate. Alternatively, an etch stop layer may be present between the phase shift layer and the substrate, in which case the second etch process will selectively stop at the etch stop layer.
Ideally, the etch process will have a high etch selectivity to both the topmost etch resistant mask (e.g., photoresist, e-beam resist, etc.) and underlying material (substrate or etch stop) while creating features that have smooth vertical side walls that exactly replicate the CD of the original mask (e.g., resist) pattern. Wet etch processes (e.g., aqueous solutions of chloric acid and ceric ammonium nitrite for AR Cr/Cr etch) show good etch selectivity to the etch mask and underlying substrate, but are isotropic and result in significant undercut of the mask which results in sloped feature profiles. The undercut and sloped feature profiles result in changes to the etched feature CD. The undesirable change in CD and/or sloped feature profiles degrade the optical performance of the finished photomask.
Dry etch (plasma) processing is a well known alternative to wet etch processing. Plasma etching provides a more anisotropic etch result than wet process. Dry etching is commonly used in the fabrication of all three mask types.
Dry etching performance is affected by a number of process factors including process gas composition and flow rate, process pressure, applied radio frequency (RF) power—both in the generation of the plasma and as an applied RF bias to the substrate, and substrate surface temperature.
A number of groups have looked at plasma etching of materials at temperatures below 25° C. Etching at reduced temperatures may eliminate undesirable trade-offs seen in near room temperature processes (e.g., improved selectivity to etch mask material while maintaining vertical etch profiles. The benefits of reduced temperature may be realized for most materials: dielectrics, semiconductors, and metals. In the case of etching chromium containing materials, a number of groups have proposed the benefits of plasma etching at reduced temperatures.
In a conventional plasma etch process, the most common method to control the substrate surface temperature is to ensure that the substrate is in intimate contact with a temperature controlled cathode and introducing a pressurized gas between the substrate and the cathode. The substrate is typically held in contact with the cathode using a clamping mechanism which may be accomplished through mechanical or electrostatic means.
In the case of mechanical clamping, a clamping ring applies sufficient force to the substrate to allow the space between the substrate and the cathode to be pressurized with a heat transfer fluid. Mechanically clamped cooling has the benefits of being insensitive to substrate material, and the ease of clamping/declamping the substrate. The limitations of mechanically clamping a substrate however are significant: for example, the presence of the clamp may adversely affect the plasma uniformity. Furthermore, the movement of the clamping mechanism and the physical contact between the clamp and the substrate are prone to cause particle generation. Finally, the area of the substrate that is contacted by the clamp is typically shielded from the plasma environment, resulting in no etching in this region. In the case of reticle fabrication by plasma etch processes, these limitations are not acceptable. Consequently, during fabrication, reticles are not typically mechanically clamped during plasma etching steps.
In an effort to overcome the limitations of mechanical clamps, electrostatic clamping was developed. Electrostatic clamps (ESC) use the attraction of opposite charges instead of a mechanical clamping ring to apply force between the substrate and the cathode. Electrostatic clamping has the advantages of being able to clamp material while only physically touching the back of the substrate. This advantage can translate into improved process uniformity and lower particle levels. However, the ESC configuration also has limitations. Residual charge on the substrate after processing can lead to residual clamping force after the ESC has been de-energized. This residual clamping force can make transferring the substrate after processing difficult. Another limitation of ESCs is their limited ability to clamp dielectric substrates. While it is possible to “clamp through” dielectric substrates to a conductive film on the front side of the substrate, thicker dielectric substrates, such as photolithographic reticles, are typically difficult to electrostatically clamp.
Note that due to the mass of the typical photolithographic substrate that it is possible to introduce a heat transfer gas at low pressures between the substrate and the cathode without clamping (less than about 1 Torr for current 150 mm×150 mm×6 mm quartz photomask substrates). While a low pressure gas will provide limited heat transfer to the substrate, the temperature of the photomask substrate will rise during exposure to plasma.
Due to the defect sensitivity of photolithographic substrates, permissible contact to photomask substrates has been historically limited to approximately the outer 10 mm of the backside of the substrate. This substrate contact constraint has precluded mechanically clamping photolithographic substrates during dry etch processing. Additionally, since the majority of photolithographic substrates are fabricated from dielectric materials (e.g., quartz), electrostatic clamping is typically difficult. Therefore, conventional plasma etch processes during reticle fabrication do not utilize substrate clamping in conjunction with backside cooling.
FIG. 1 shows a typical substrate stage configuration for a photolithographic dry etch system used in reticle fabrication. The substrate 105 is placed on a support cover plate 115. The cover plate 115 may be in thermal contact with the substrate support 120 or thermally isolated. The support cover plate 115 rests on the substrate support 120. The cover plate 115 typically contains a recess that accommodates the substrate 105 such that the top surface of the substrate 105 and cover plate 115 are approximately coplanar. The cover plate 115 contacts the substrate 105 only on the outer edge 110 of the back surface of the substrate 105. The region of contact of the back of the substrate 105 is typically within than the outer 10 mm on the back surface of the substrate 105. The contact between the substrate 105 and the cover plate 115 may be a continuous ledge, point contacts, or some combination thereof. Since the coverplate 115 only contacts the substrate 105 at the outer edges 110 of the rear face, there is typically a thin gap 100 between the back face of the substrate 105 and the substrate support 120.
While the temperature of the substrate support 120 is controlled during the process through contact with a heat transfer fluid (not shown) there is only limited heat transfer between the substrate 105 and the coverplate 115. Therefore, in the absence of helium backside cooling, the photolithographic substrate 105 is subject to heating by the plasma during the dry etch process. The rate of heating during the process is a function of the process parameters, including the RF powers, chamber wall temperatures, etc. The photolithographic substrate 105 is typically not actively cooled during the dry etch process. Consequently, the temperature of the substrate 105 will increase during the time it is exposed to the plasma.
Based on the prior art, there is a need for an improved method to control the temperature of a photolithographic substrate prior to plasma etching during the reticle fabrication process.
Nothing in the prior art provides the benefits attendant with the present invention.
Therefore, it is an object of the present invention to provide an improvement which overcomes the inadequacies of the prior art devices and which is a significant contribution to the advancement to the processing of photomasks and reticles.
Another object of the present invention is to provide a method for processing a photolithographic substrate, comprising placing said photolithographic substrate on a support member in a chamber, said photolithographic substrate having an initial temperature; introducing a heat transfer fluid into said chamber; cooling said photolithographic substrate through said heat transfer fluid to a target temperature; and subjecting said cooled photolithographic substrate to a plasma process before the temperature of said cooled photolithographic substrate reaches said initial temperature.
Yet another object of the present invention is to provide a method for processing a photolithographic substrate, comprising placing said photolithographic substrate on a first support member in a first chamber; introducing a heat transfer fluid into said first chamber; cooling said photolithographic substrate in said first chamber through said heat transfer fluid to a first process set point; transferring said cooled photolithographic substrate out of said first chamber on to a second support member in a second chamber; and subjecting said cooled photolithographic substrate to a plasma process in said second chamber before the temperature of said cooled photolithographic substrate reaches a second process set point.
The foregoing has outlined some of the pertinent objects of the present invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.